Heart failure (HF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body's tissues. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow may become leaky, allowing regurgitation or backflow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness, and inability to carry out daily tasks may result.
Not all HF patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As HF progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output.
Current standard treatment for HF is typically centered around medical treatment using ACE inhibitors, diuretics, and digitalis. It has also been demonstrated that aerobic exercise may improve exercise tolerance, improve quality of life, and decrease symptoms. Cardiac surgery has also been performed on a small percentage of patients with particular etiologies. Although advances in pharmacological therapy have significantly improved the survival rate and quality of life of patients, some HF patients are refractory to drug therapy, have a poor prognosis and limited exercise tolerance. In recent years cardiac pacing, in particular Cardiac Resynchronization Therapy (CRT), has emerged as an effective treatment for many patients with drug-refractory HF.
HF patients require close medical management to reduce morbidity and mortality. Because the disease status evolves with time, frequent physician follow-up examinations are often necessary. At follow-up, the physician may make adjustments to the drug regimen in order to optimize therapy. This conventional approach of periodic follow-up may be less satisfactory for HF, in which acute, life-threatening exacerbations can develop between physician follow-up examinations. It is well known among clinicians that if a developing exacerbation is recognized early, it can be more easily and inexpensively terminated, typically with a modest increase in oral diuretic. However, if it develops beyond the initial phase, an acute HF exacerbation becomes difficult to control and terminate. Hospitalization in an intensive care unit is often required. It is during an acute exacerbation of HF that many patients succumb to the disease. Early identification may also allow for pacing therapy from an implanted pulse generator.
In view of the above, it would be beneficial if a patient's HF condition can be chronically monitored. Additionally, it would be beneficial if therapy can be automatically delivered and adjusted to improve HF treatment. Further, it would be beneficial to facilitate the early termination of a developing HF exacerbation.
A properly time ordered ventricular electrical activation sequence and proper inter-ventricular and intra-ventricular delays (which can collectively be referred to as a proper ventricular electrical activation pattern) enable efficient contractions of the ventricular chambers of the heart, thereby contributing to efficient blood perfusion through both the lungs and the systemic circulation. However, in patients with right bundle branch block (RBBB), left bundle branch block (LBBB), or other ventricular conduction abnormalities, their ventricular electrical activation sequence and inter-ventricular and intra-ventricular delays are often adversely affected, which may adversely affect blood perfusion and, more generally, may adversely affect cardiac mechanical function. Accordingly, it would be desirable to be able to detect when a patient's ventricular electrical activation pattern deviates from a predetermined preferred pattern, and to increase compliance with the predetermined preferred pattern.
In a normal heart, cells of the sinoatrial node (SA node) spontaneously depolarize and thereby initiate an action potential. This action potential propagates rapidly through the atria (which contract), slowly through the atrioventricular node (AV node), the atrioventricular bundle (AV bundle or His bundle) and then to the ventricles, which causes ventricular contraction. This sequence of events is known as normal sinus rhythm (NSR). Thus, in a normal heart, ventricular rhythm relies on conduction of action potentials through the AV node and AV bundle.
Rhythms that do not follow the sequence of events described above are known as arrhythmias. Those that result in a heart rate slower than normal are known as bradyarrhythmias; those that result in a faster heart rate than normal are called tachyarrhythmias. Tachyarrhythmias are further classified as supraventricular tachyarrhythmias (SVTs) and ventricular tachyarrhythmia (VT). SVTs are generally characterized by abnormal rhythms that may arise in the atria or the atrioventricular node (AV node). Additionally, there are various types of different SVTs and various types of VTs that can be characterized. For example, a paroxysmal SVT can exhibit heart rates between approximately 140 beats per minute (bpm) and approximately 250 bpm. However, the most common SVTs are typically atrial flutter (AFL) and atrial fibrillation (AF). In addition, many SVTs involve the AV node, for example, AV nodal reentry tachycardia (AVNRT) where an electrical loop or circuit includes the AV node. Another type of SVT is an AV reentrant tachycardia (AVRT), where an AV reentrant circuit typically involves the AV node and an aberrant conducting bundle known as an accessory pathway that connects a ventricle to an atrium.
Atrial flutter (AFL) can result when an early beat triggers a “circus circular current” that travels in regular cycles around the atrium, pushing the atrial rate up to approximately 250 bpm to approximately 350 bpm. The atrioventricular node between the atria and ventricles will often block one of every two beats, keeping the ventricular rate at about 125 bpm to about 175 bpm. This is the pulse rate that will be felt, even though the atria are beating more rapidly. At this pace, the ventricles will usually continue to pump blood relatively effectively for many hours or even days. A patient with underlying heart disease, however, may experience chest pain, faintness, or even HF as a result of the continuing increased stress on the heart muscle. In some individuals, the ventricular rate may also be slower if there is increased block of impulses in the AV node, or faster if there is little or no block.
If the cardiac impulse fails to follow a regular circuit and divides along multiple pathways, a chaos of uncoordinated beats results, producing AF. AF commonly occurs when the atrium is enlarged (usually because of heart disease). In addition, it can occur in the absence of any apparent heart disease. In AF, the atrial rate can increase to more than 350 bpm and cause the atria to fail to pump blood effectively. Under such circumstances, the ventricular beat may also become haphazard, producing a rapid irregular pulse. Although AF may cause the heart to lose approximately 20 to 30 percent of its pumping effectiveness, the volume of blood pumped by the ventricles usually remains within the margin of safety, again because the atrioventricular node blocks out many of the chaotic beats. Hence, during AF, the ventricles may contract at a lesser rate than the atria, for example, of approximately 125 bpm to approximately 175 bpm.
Overall, SVTs are not typically immediately life threatening when compared to ventricular arrhythmias, examples of which are discussed below.
Ventricular arrhythmias, which originate in the ventricles, include ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular arrhythmias are often associated with rapid and/or chaotic ventricular rhythms. For example, sustained VT can lead to VF. In sustained VT, consecutive impulses arise from the ventricles at a rate of 100 bpm or more. Such activity may degenerate further into disorganized electrical activity known as ventricular fibrillation (VF). In VF, disorganized action potentials can cause the myocardium to quiver rather than contract. Such chaotic quivering can greatly reduce the heart's pumping ability. Indeed, approximately two-thirds of all deaths from arrhythmia are caused by VF. A variety of conditions such as, but not limited to, hypoxia, ischemia, pharmacologic therapy (e.g., sympathomimetics), and asynchronous pacing may promote onset of ventricular arrhythmia. Further, there are various different types of VT, including, e.g., monomorphic VT and polymorphic VT, for which different types of therapy may be appropriate.
It has been common practice for an implantable cardioverter defibrillator (ICD) to monitor heart rate, or more commonly the ventricular rate, of a patient and classify the cardiac condition of the patient based on this heart rate. For example, a tachyarrhythmia may be defined as any rate in a range above a designated threshold. This range is then divided into ventricular tachycardia and ventricular fibrillation zones. The ventricular tachycardia zone may be further divided into slow ventricular tachycardia and fast ventricular tachycardia zones.
As described above, both SVTs and ventricular arrhythmias may lead to ventricular rates in excess of 100 bpm. In other words, ventricular rates of SVTs can overlap with rates of tachycardias of ventricular origin. These SVTs are often well tolerated and require no intervention. Further, physically active patients can have heart rates during exercise that overlap with their tachycardia rates. Accordingly, discrimination of VT from SVT, including increased heart rates due to exercise, may require more than just knowledge of a patient's ventricular rate. In other words, using heart rate as the sole criterion to classify the cardiac condition of a patient is often not sufficient.
To improve the specificity and accuracy of arrhythmia characterization, many ICDs can also examine the morphology of an intracardiac electrogram (IEGM), in addition to the heart rate. The shape of an intracardiac complex can include information on the origin and sequence of the heart's electrical activity. A normal intracardiac complex traverses the AV node and is conducted by specialized cardiac tissue throughout the ventricles. This results in a distinctive complex morphology. A tachycardia of ventricular origin often has a different morphology due to its ectopic origin and conductance through cardiac muscle tissue. As such, in addition to monitoring heart rate, some ICDs are capable of performing morphology discrimination to classify the cardiac condition of the patient. For example, a template based on the morphology of a “known” signal can be stored in the ICD. The “known” signal can be, for example, a signal collected during a period where a patient is known to exhibit a normal sinus rhythm. By comparing the morphology characteristics (e.g., number, amplitude, sequence and/or polarity of waveform peaks, as well as the area of the peaks) of an arrhythmia to the template, the ICD can calculate the match (or lack thereof) between the waveforms. For a further description of morphology discrimination, refer to U.S. Pat. No. 5,240,009 (Williams), entitled “Medical Device with Morphology Discrimination” and to U.S. Pat. No. 5,779,645 (Olson et al.) entitled “System and Method for Waveform Morphology Comparison,” which patents are hereby incorporated by reference. These are just a few examples of morphology discriminator algorithms and parameters, which are not intended to be limiting.
Sudden onset and interval stability (also know as rate stability), are examples of other factors that can be monitored to improve the specificity of arrhythmia characterization. Also, the relationship between ventricular rate (V) and atrial rate (A) can be used to characterize an arrhythmia. For example, this can be part of a rate branch algorithm, which, depending on V and A, may follow one of three branches: a V<A rate branch; a V=A (within a specified tolerance) rate branch; and a V>A rate branch. If V<A, then morphology discrimination and/or interval stability may be available to distinguish VT from AF or AFL. If A and V are essentially the same (within a certain tolerance), then morphology discrimination and/or sudden onset may be available to distinguish VT from sinus tachycardia. If V>A, then an arrhythmia may be characterized as VT. Also, specific branches can be turned on or off. For example, if V is greater than the tachycardia threshold but essentially the same as A, and the V=A branch is turned off, then the algorithm can cause the V>A branch to be followed, and the arrhythmia may be classified as VT. Additional details of an exemplary rate branch algorithm are provided in U.S. Pat. No. 6,636,764 (Fain et al.), entitled “Safety Backup in Arrhythmia Discrimination Algorithm,” which is incorporated herein by reference. Also, atrioventricular association (AVA) can also be used to distinguish AFL from VT. In an exemplary AVA algorithm, the AV interval is measured from each ventricular sensed event to its preceding atrial event and an AVA Delta is then calculated as the difference between the second longest AV interval and the second shortest AV interval in a recent group of intervals. If the measured AVA Delta is less than a programmable AVA threshold parameter, the AV intervals are considered stable, which is indicative of SVT. If the measured AVA Delta is greater than or equal to a programmable AVA threshold parameter, the AV intervals are considered unstable, which is indicative of VT. More generally, the relative rate of the atria and ventricles and/or the timing relationship between atrial and ventricular events can be considered.
Typically an ICD is programmed to provide a therapy in response to an arrhythmia being detected, where the type of therapy corresponds to the type of arrhythmia that the ICD believes it has detected. For example, VT may be treated with a therapy consisting of low-energy pacing pulses designed to capture the ventricles. This therapy is referred to as ventricular anti-tachycardia pacing therapy (V-ATP). VT may also be treated with relatively low energy, synchronized cardioversion shocks. VF, on the other hand, is typically treated more aggressively with high energy shocks. SVT may not be treated, or may be treated using atrial ATP (A-ATP) or atrial defibrillation. Quite often, SVT is treated using medication, or ablation.
Inappropriate therapy is a huge problem for ICD patients. Inappropriate therapies, specifically inappropriate shocks cause great suffering among the ICD patient population. Patients receive a potentially life-saving device (the ICD) only to find out that it sometimes malfunctions and inflicts both pain and harm without any warning whatsoever. For a secondary prevention patient, this might be bearable since he or she has experienced and been saved from a lethal tachyarrhythmia in the past. For the primary prevention patient, however, without a history of arrhythmias who receives the ICD only based on a risk score, this is more difficult to bear.
Today only a few of the patients who are candidates for an ICD receive one. This is mostly an economic issue; although several studies indicate that an ICD is a cost effective treatment in indicated patients. However, if the performance of the devices improves, the willingness to put in an ICD, especially in young primary prevention patients, will increase.
Despite the numerous arrhythmia discrimination techniques that exist, examples of which were provided above, delivery of inappropriate therapy remains a major problem with ICDs today. For example, a common cause of inappropriate shocks in ICD devices are atrial arrhythmias that are conducted to the ventricles at a high rate. Accordingly, there is still a need for new, and preferably improved, arrhythmia discrimination techniques.